1. Field of the Invention
The present invention relates to an image reading apparatus for reading an original image using an image sensor.
2. Related Background Art
As conventional image reading apparatuses for reading an original image using a linear CCD (charge coupled device) image sensor, an apparatus which has an ABC (Auto Back-ground Control) circuit for automatically varying the read signal in correspondence with the brightness of an original, is known.
FIG. 17 shows an example of the arrangement of the conventional image reading apparatus with such ABC circuit (to be referred to as a first conventional apparatus hereinafter). In FIG. 17, a B/W (black/white) CCD image sensor 1801 converts an optical signal coming from an optical system (not shown) into an electrical signal, and outputs it as an image signal. An amplifier 1802 amplifies the output from the CCD image sensor 1801 to a predetermined level. An A (analog)/D (digital) conversion circuit 1803 converts the output from the amplifier 1802 into a digital signal. A comparator 1804 compares the digital image signal output from the A/D conversion circuit 1803 with a predetermined reference level for ABC comparison. An LPF (low-pass filter) 1805 removes (cuts) components in the high-frequency range (high-frequency components) of the output signal from the comparator 1804. A reference voltage source 1806 generates a reference voltage VREF to be input to VB (bottom reference) of VT (top reference) and VB (bottom reference), which determine the input range of the A/D conversion circuit 1803.
In FIG. 17, the output signal from the comparator 1804, from which high-frequency components have been cut by the LPF 1805, is input to the VT (top reference) terminal of the A/D conversion circuit 1803. For this reason, when the signal level of the digital image signal output from the A/D conversion circuit 1803 is larger than the reference level for ABC comparison, the VT (top reference) of the A/D conversion circuit 1803 is controlled to increase, and consequently, the signal level of the digital image signal output from the A/D conversion circuit 1803 decreases. By contrast, when the signal level of the digital image signal becomes smaller than the reference level for ABC comparison, the VT (top reference) of the A/D conversion circuit 1803 is controlled to decrease, and as a result, the signal level of the digital image signal output from the A/D conversion circuit 1803 increases.
As a result of the control, the average level of the digital image signal output from the A/D conversion circuit 1803 matches the reference level for ABC comparison.
In FIG. 17, the comparator 1804 receives an area signal for ABC, so that the comparator 1804 outputs a comparison signal in only an image area to be subjected to ABC comparison, but stops its output in an image area (e.g., a non-image period and image edge portions) not to be subjected to ABC comparison.
Hence, in the first conventional apparatus, since the read signal level is controlled so that the average level obtained by reading an original image always matches the reference level for comparison, both dark and light original images can always be read with optimal signal levels.
FIG. 18 shows an example of the arrangement of another image reading apparatus using a linear image sensor (to be referred to as a second conventional apparatus hereinafter). In this second conventional apparatus, an original 1911 on a platen glass 1910 is illuminated using a light source 1909 for illuminating an original, and a reflection member 1908, and an optical original image is formed on the light-receiving surface of a CCD linear image sensor 1901 via first, second, and third mirrors 1907, 1904, and 1905, and a lens 1902. A first moving unit 1906 bounded by the one-dashed chain line in FIG. 18 moves at a speed V in the direction of an arrow A in FIG. 18, and a second moving unit 1903 bounded by the two-dashed chain line moves at a speed V/2 in the direction of an arrow B in FIG. 18 (to be referred to as a subscan hereinafter) at the same time, thus reading the image on the entire original 1911 by the CCD linear image sensor 1901.
FIG. 19 shows an example of the flow of signal processing of an image signal obtained from the CCD linear image sensor 1901 shown in FIG. 18. In FIG. 19, the output signal from the CCD linear image sensor 1901 is amplified by a gain amplifier 2001, and is input to an analog signal processing circuit 2002 to extract an image signal component. The analog image signal is converted by an A (analog)/D (digital) conversion circuit 2003 into a digital image signal. The digital image signal is subjected to shading correction for uniformly reading an original image in a shading correction circuit 2004. After that, the digital image signal is output from an output terminal 2005.
In still another conventional image reading apparatus (to be referred to as a third conventional apparatus hereinafter), since the reading speed of an original image is low like in facsimile apparatuses and the like, a photoelectric conversion element for one line can correspond to a single A/D converter. For this reason, the individual differences (variations in characteristics) of A/D converters upon processing one-line data of an original using different A/D converters need not be taken into consideration.
In recent years, demand has arisen for mounting an ABC circuit on the above-mentioned first conventional apparatus in a color image reading apparatus using a color CCD image sensor.
However, when the arrangement of the ABC circuit is used in the color image reading apparatus, the following problem is posed.
That is, when independent ABC circuits are added for the read signals of the respective colors read by the color image reading apparatus, the reading levels of the respective colors are independently controlled. For this reason, the color tone of the read image has a large difference from that of the original.
In the flow of the signal processing shown in FIG. 19 in the second conventional apparatus, when the original has a color background or a background pattern, and characters are printed thereon, if such original is read by the image reading apparatus, since the level difference between the background and characters is small, the characters may become unclear when the read original image is printed, output to a TV monitor, or output as a facsimile image.
In the third conventional apparatus, when an apparatus with high work efficiency is to be constructed without impairing productivities of the individual functions in a multi-functional image forming apparatus, some of the functions that can operate even at a low reading speed of an original image are required to have a high reading speed of an original image so as to optimize productivities. In an image reading unit, since the photoelectric conversion/transfer speed of the photoelectric conversion element serving as a reading means has an upper limit, photocells which line up in the main scan direction are divided into some groups using their even- and odd-number column positions to attain parallel signal processing, thereby apparently increasing the reading speed. However, when background removal (processing for forcibly recognizing the background as a white background) for an original to be fed and read, the original cannot be read again. Hence, real-time processing is inevitably required, but is impossible to attain in practice.